A simplified scheme for realizing multi-atom NOON state

We present a scheme for realizing a multi-atom NOON state via cavity QED system. The scheme bases on the Jaynes-Cumming mode with collective atomic bosonic mode. In the process, a series of control atoms are sent through two single mode cavities which are initially in vacuum states and have the same collective atoms. After the suitable interaction time, the collective atoms in two cavities are in the desired state.     

Scheme for implementing frequency up-conversion between two collective atomic modes

This paper presents a scheme for realizing the frequency up-conversion between two collective atomic modes. In the scheme two atomic samples are coupled to a cavity mode. Under the large detuning condition, the two collective atomic modes are coupled via the virtual excitation of the cavity mode and the effective Hamiltonian corresponds to the frequency up-conversion. In the scheme the cavity mode is only virtually excited and thus the process is insensitive to cavity decay.     

Forces and spatial ordering of driven atoms in a resonator in the regime of fluorescence suppression

An atom in a high-Q cavity, which is coherently driven at the frequency of a cavity mode, exhibits strong suppression of fluorescence when the atomic decay rate exceeds the cavity linewidth. This effect is due to destructive interference of cavity and pump field, such that at the atomic position the total field intensity has a local minimum. For atomic ensembles the magnitude of the interference effect grows with atom number and depends on the relative atomic positions. It is strongest for a wavelength spaced array of atoms placed at the antinodes of the cavity mode. This suppresses fluorescence and enhanced collective scattering into the cavity mode. We analyze the mechanical forces in the regime where the interference condition is fulfilled. We show that the atomic pattern is mechanically stable whenever the driving frequency is red detuned with respect to the cavity frequency, irrespective of the atomic transition frequency. Hence atomic selforganization, as predicted in [6] can also occur in the parameter regime where superradiant scattering is suppressed by collective interference. PACS 32.80.Pj; 42.50.Pq; 42.50.Fx     

Optomechanical cavity cooling of an atomic ensemble

We demonstrate cavity sideband cooling of a single collective motional mode of an atomic ensemble down to a mean phonon occupation number ?n?(min?)=2.0(-0.3)(+0.9). Both ?n?(min) and the observed cooling rate are in good agreement with an optomechanical model. The cooling rate constant is proportional to the total photon scattering rate by the ensemble, demonstrating the cooperative character of the light-emission-induced cooling process. We deduce fundamental limits to cavity cooling either the collective mode or, sympathetically, the single-atom degrees of freedom.     

Rotonlike Fulde-Ferrell collective excitations of an imbalanced Fermi gas in a two-dimensional optical lattice

We address the question of whether superfluidity can survive in the case of fermion pairing between different species with mismatched Fermi surfaces using as an example a population-imbalanced mixture of 6Li atomic Fermi gas loaded in a two-dimensional optical lattice at nonzero temperatures. The collective mode is calculated from the Bethe-Salpeter equations in the general random phase approximation assuming a Fulde-Ferrell order parameter. The numerical solution shows that, in addition to low-energy (Goldstone) mode, two rotonlike minima exist, and therefore, the superfluidity can survive in this imbalanced system.     

Superradiance and Collective Gain in the Atom-Assisted Multimode Optomechanical System

We investigate that the muti-mode optomechanical system coupled with the two-level atoms. If the driving pump field is resonance with the anti-Stokes sideband, the system is at the superradiative state. For the driving filed in the Stokes sideband, the collective gain can be observed. We study a scheme that how the atomic medium affect these superradiance and collective gain. Our results show that the presence of the atom can enhance the superradiant behavior. In the mode splitting regime, the mode splits into thirds with the presence of the atoms with the anti-Stokes sideband. In addition, we also show that the use of atoms in this system could provide us a way to switch the system form superradiative state to collective gain.

     

Adiabatic Passage of Collective Excitations in Atomic Ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Ramann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.     

Adiabatic Passage of Collective Excitations in Atomic Ensembles

We describe a theoretical scheme that allows for transfer of quantum states of atomic collective excitation between two macroscopic atomic ensembles localized in two spatially-separated domains. The conception is based on the occurrence of double-exciton dark states due to the collective destructive quantum interference of the emissions from the two atomic ensembles. With an adiabatically coherence manipulation for the atom-field couplings by stimulated Rmann scattering, the dark states will extrapolate from an exciton state of an ensemble to that of another. This realizes the transport of quantum information among atomic ensembles.     

Interfacing collective atomic excitations and single photons

We study the performance and limitations of a coherent interface between collective atomic states and single photons. A quantized spin-wave excitation of an atomic sample inside an optical resonator is prepared probabilistically, stored, and adiabatically converted on demand into a sub-Poissonian photonic excitation of the resonator mode. The measured peak single-quantum conversion efficiency of chi=0.84(11) and its dependence on various parameters are well described by a simple model of the mode geometry and multilevel atomic structure, pointing the way towards implementing high-performance stationary single-photon sources.     

End and central plasmon resonances in linear atomic chains

The existence and nature of end and central plasmon resonances in a linear atomic chain, the 1D analog to surface and bulk plasmons in 2D metals, has been predicted by ab initio time-dependent density functional theory. Length dependence of the absorption spectra shows the emergence and development of collectivity of these resonances. It converges to a single resonance in the longitudinal mode, and two transverse resonances, which are localized at the ends and center of the atom chains. These collective modes bridge the gaps, in concept and scale, between the collective excitation of atomic physics and nanoplasmonics. It also outlines a route to atomic-scale engineering of collective excitations.     

Conditional quantum phase gate between two 3-state atoms

We propose a scheme for conditional quantum logic between two 3-state atoms that share a quantum data bus such as a single mode optical field in cavity QED systems, or a collective vibrational state of trapped ions. Making use of quantum interference, our scheme achieves successful conditional phase evolution without any real transitions of atomic internal states or populating the quantum data bus. In addition, it requires only common addressing of the two atoms by external laser fields.     

Dielectric properties of multiband electron systems II. Collective modes

Starting from the tight-binding dielectric matrix in the random phase approximation we examine the collective modes and electron-hole excitations in a two-band electronic system. For long wavelengths (q → 0), for which most of the analysis is carried out, the properties of the collective modes are closely related to the symmetry of the atomic orbitals involved in the tight-binding states. In insulators there are only inter-band charge oscillations. If atomic dipolar transitions are allowed, the corresponding collectivemodes reduce in the asymptotic limit of vanishing bandwidths to Frenkel excitons for an atomic insulator with weak on-site interactions. The finite bandwidths renormalize the dispersion of these modes and introduce a continuum of incoherent inter-band electron-hole excitations. The possible Landau damping of collective modes due to the presence of this continuum is discussed in detail. In conductors the intra-band charge fluctuations give rise to plasmons. If the atomic dipolar transitions are forbidden, the coupling of inter-band collective modes and plasmons tends to zero as q → 0. On the contrary, in dipolar conductors this coupling is strong and nonperturbative, due to the long range monopole-dipole interactions between intra-band and inter-band charge fluctuations. The resulting collective modes are hybrids of intra-band plasmons and inter-band dipolar oscillations. It is shown that the frequency of the lower hybridized longitudinal mode is proportional to the frequency of the transverse dipolar mode when the latter is small. The dielectric instability in a multi-band conductor is therefore characterized by the simultaneous softening of a transverse and a longitudinal mode, which is an important, directly measurable consequence of the present theory.     

Spectroscopy of superfluid pairing in atomic fermi gases

We study the dynamic structure factor for density and spin within the crossover from BCS superfluidity of atomic fermions to the Bose-Einstein condensation of molecules. Both structure factors are experimentally accessible via Bragg spectroscopy and allow for the identification of the pairing mechanism: the spin structure factor allows for the determination of the two particle gap, while the collective sound mode in the density structure reveals the superfluid state.     

核子与核力决定的核形状(英文)

讨论了最近提出的作为量子多体系统重要潜在机制之一的量子自组织,原子核无疑是最好的实例。由于原子核内核子的单粒子和集体运动共存,它们的相互制约决定了核结构。集体模式因其驱动力,如使椭球形变的四极力及其阻力达到平衡形成,而单粒子能量就是产生阻力的一种根源。当存在较大单粒子能隙时,相关的集体运动更易受到阻碍。因此,一般认为,单粒子运动和集体运动是相互对抗的"天敌"。然而,由于核力的多样和复杂性,单极相互作用使单粒子能量改变也能减小其对集体运动的阻碍而加强集体模式,该现象将通过Zr同位素实例加以说明。这就导致了量子自组织的产生:单粒子能量由两种量子液体（质子和中子）和控制阻力的单极相互作用自组织。于是,不同于朗道费米液体理论的结论,原子核不一定像填装了自由核子的刚性瓶。Ⅱ型壳演化即是包含跨准幻壳能隙激发的直观实例。在重核中,量子自组织因其轨道和核子数更多而更为重要。We discuss the quantum self-organization introduced recently as one of the major underlying mechanisms of the quantum many-body systems. Atomic nuclei are actually a good example, because two types of the motion of nucleons, single-particle states and collective modes, interplay in determining their structure. The collective mode appears as a consequence of the balance between the effect of the mode-driving force (e.g., quadrupole force for the ellipsoidal deformation) and the resistance power against it. The single-particle energies are one of the sources to bring about such resistance power:a coherent collective motion is more hindered by larger spacings between relevant single particle states. Thus, the single-particle state and the collective mode are "enemies" against each other in the usual understanding. However, the nuclear forces are rich and complicated enough so as to enhance relevant collective mode by reducing the resistance power by changing single-particle energies for each eigenstate through monopole interactions. This will be demonstrated with the concrete example taken from Zr isotopes. In this way, the quantum self-organization occurs:single-particle energies can be self-organized by (i) two quantum liquids, e.g., protons and neutrons, (ii) monopole interaction (to control resistance). Thus, atomic nuclei are not necessarily like simple rigid vases containing almost free nucleons, in contrast to the naïve Fermi liquid picture a la Landau. Type Ⅱ shell evolution is considered to be a simple visible case involving excitations across a (sub)magic gap. The quantum self-organization becomes more important in heavier nuclei where the number of active orbits and the number of active nucleons are larger.     

Optically-driven cooling for collective atomic excitations

We explore how to cool collective atomic excitations in an optically-driven three-level atomic ensemble, which may be described by a model of two coupled harmonic oscillators (HOs) with a time-dependent coupling. Moreover, the model of two coupled HOs is further generalized to address the resolved sideband cooling issues, where the lower-frequency HO can be cooled whenever the cooling process dominates over the heating one during the sideband transitions. Unusually, due to the absence of the heating process, the optimal result for cooling collective excitations in an atomic ensemble could break the standard resolved sideband cooling limit for general models of two coupled HOs.     

Enhancing the capacity and performance of collective atomic quantum memory

Present schemes involving the quantum non-demolition interaction between atomic samples and off-resonant light pulses allow us to store quantum information corresponding to a single harmonic oscillator (mode) in one multi-atomic system. We discuss the possibility of involving several coherences of each atom so that the atomic sample can store information contained in several quantum modes. This is achieved by the coupling of different magnetic sublevels of the relevant hyperfine level by additional Raman pulses. This technique allows us to design not only the quantum non-demolition coupling, but also beam splitter-like and two-mode squeezer-like interactions between light and collective atomic spin.     

Supersymmetry and the Goldstino-like mode in bose-fermi mixtures

Supersymmetry is assumed to be a basic symmetry of the world in many high-energy theories, but none of the superpartners of any known elementary particle have been observed yet. We argue that supersymmetry can also be realized and studied in ultracold atomic systems with a mixture of bosons and fermions, with properly tuned interactions and single particle dispersion. We further show that in such nonrelativistic systems supersymmetry is either spontaneously broken or explicitly broken by a chemical potential difference between the bosons and fermions. In both cases the system supports a sharp fermionic collective mode similar to the Goldstino mode in high-energy physics, due to supersymmetry. We also discuss possible ways to detect this mode experimentally.     

A scheme of quantum phase gate for trapped ion

We propose a scheme to implement two-qubit controlled quantum phase gate(CQPG) via a single trapped two-level ion located in the standing wave field of a quantum cavity, in which the trap works beyond the Lamb--Dicke limit. When the light field is resonant with the atomic transition $|g\rangle\leftrightarrow|e\rangle$ of the ion located at the antinode of the standing wave, we can perform CQPG between the internal and external states of the trapped ion; while the frequency of the light field is chosen to be resonant with the first red sideband of the collective vibrational mode of the ion located at the node of the standing wave, we can perform CQPG between the cavity mode and the collective vibrational mode of the trapped ion. Neither the Lamb--Dicke approximation nor the assistant classical laser is needed. Also we can generate a GHZ state if assisted with a classical laser.     

Continuous-variable multimode entanglement in multi-wave mixing

We present a scheme to obtain continuous-variable multimode entanglement via multi-wave mixing. For a four-level atomic system in a diamond configuration, four strong coherence fields are applied to the four dipole-allowed transitions to amplify eight sidebands as cavity fields, respectively. Due to the atomic coherence, the eight side modes constitute a pair of double quantum-beats and combine into two quantum-beat collective modes, which are in a four-wave mixing parametric interaction. As a result, the entanglement occurs between these two collective modes. Correspondingly, any two individual modes from two different collective modes are entangled with each other. This gives rise to continuous-variable eight-mode entanglement with different frequencies, which has extensively application in quantum communication and quantum network.     

Quantum-trajectory approach to cavity quantum electrodynamics with up to three-atom collective effects

We present a quantum-trajectory treatment of the dynamics of a high-Q cavity mode interacting with a low-density atomic beam in the strong coupling regime. We consider up to three-atom collective effects, and evaluate their contribution to the destabilization of cavity field trapping states for mean number of atoms in the cavity both much smaller than one (micromaser or microlaser), and on the order of one (approaching a mesoscopic regime).